But why isn't it possible to engineer a vigorous piston effect, above and beyond mere exhaust impingement? Like a gas piston, or a bullet launching from a gun barrel. The idea would be for the separating stages to push away from each other with the highest possible impulse.

wrong. Because

1. the upper stages, full of propellants and payload, are likely heavier than the discarded lower stage. It would a more velocity to the lower stage than the upper stage.

2. The interstage would have to be reinforced to handle the pressure so that it could be harnessed. This would increase mass more than any benefit derived from the impulse. Titan II did every thing it could to release the pressure. That is why Russian vehicles have lattice interstages. The pr

Do the newer methods, like the pusher-rod on Falcon 9, offer the highest possible impulse at separation?

no

The point is not impulse but to provide separation. The pusher rod is not accelerating the upper stage. It is just pushing it clear. The design does not look at staging as a place to get delta V out of the event, but just to provide clearance from the lower stage.

Hi - I'm just going to change the thread title to "Stage Separation (Was: Hot-Staging)", if you all don't mind - just for a little broader about separation dynamics, and the tradeoffs of different methods.

But why isn't it possible to engineer a vigorous piston effect, above and beyond mere exhaust impingement? Like a gas piston, or a bullet launching from a gun barrel. The idea would be for the separating stages to push away from each other with the highest possible impulse.

wrong. Because

1. the upper stages, full of propellants and payload, are likely heavier than the discarded lower stage. It would a more velocity to the lower stage than the upper stage.

But even that Delta-V for lower stage can be beneficial in the case where lower stage wants to do RTLS (eg. F9R booster) Then the boostback requirement could be reduced.

Furthermore, why couldn't the mass balance between upper and lower stage be shifted to help momentum exchange favor a higher proportion of Delta-V for upper stage?Take the case of a winged flyback booster, which features the added mass of wings for the purpose of reusability on the lower stage.

2. The interstage would have to be reinforced to handle the pressure so that it could be harnessed. This would increase mass more than any benefit derived from the impulse. Titan II did every thing it could to release the pressure. That is why Russian vehicles have lattice interstages.

In the winged flyback booster example I gave above, the nose of the booster might already have to be reinforced for re-entry purposes anyway.

Do the newer methods, like the pusher-rod on Falcon 9, offer the highest possible impulse at separation?

no

The point is not impulse but to provide separation. The pusher rod is not accelerating the upper stage. It is just pushing it clear. The design does not look at staging as a place to get delta V out of the event, but just to provide clearance from the lower stage.

Fine - I get that - the main concern has always been about getting the upper stage clear of the lower one, so that it could continue on unhindered.

But surely stage separation can accomplish more than just that, can't it?Has it ever been considered?

1. But even that Delta-V for lower stage can be beneficial in the case where lower stage wants to do RTLS (eg. F9R booster) Then the boostback requirement could be reduced.

2. Furthermore, why couldn't the mass balance between upper and lower stage be shifted to help momentum exchange favor a higher proportion of Delta-V for upper stage?

3. In the winged flyback booster example I gave above, the nose of the booster might already have to be reinforced for re-entry purposes anyway.

4. But surely stage separation can accomplish more than just that, can't it?

1. No, because the velocity to be gained is insignificant

2. no, because the stages are sized for other reasons that have magnitudes more effect on performance

3. So you are proposing an winged flyback booster with an open cylinder for a nose?

4. No, see #1 and because any changes would have a negative effect. The duration of the separation effect is so short there is no need to look for any gains, the point is to minimize losses. The best thing is to get the upperstage away from the booster and start operating. You aren't going to get any thing more efficient than operating the upperstage. Anything else is adding mass and would basically have less equivalent performace than the upperstage or the booster.

Here is the puzzle. How would adding pistons, pushers, reinforced interstage, etc provide more velocity than the equivalent mass of propellant in the upperstage?